Recently, the patterns of integrated circuits produced on a wafer have become finer with the increasing degree of integration in LSIs; furthermore, mass production of super LSIs having super-fine patterns of quarter micrometers (0.25 .mu.m) or less in fineness is now under way. To obtain such super-fine patterns, it is also necessary to use exposure light sources having a shorter wavelength, and steppers using excimer laser radiation as the light source have been developed. The steppers equipped with KrF excimer laser radiation (248 nm in wavelength) as the light source have already been put to practical use. Furthermore, steppers equipped with ArF excimer laser radiation (193 nm in wavelength) as the light source are attracting much attention as a promising stepper of the next generation. As a glass material which exhibits sufficiently high transmittance in the short wavelength region of the KrF excimer laser and ArF excimer laser radiations, there can be mentioned a quartz glass, fluorite, etc. Particularly among them, a synthetic quartz glass prepared by fusion and vitrification of a product obtained by flame hydrolysis of a high purity silicon compound and the like is preferred as an optical material for lithography using excimer laser radiation as the light source, because it exhibits high transmittance in the short wavelength region of 260 nm or less.
In the case of employing the synthetic quartz glass mentioned above as an optical materials for use in lithography using excimer laser radiation as the light source, and particularly, in the case of using ArF excimer laser radiation, it is required that the glass has an internal transmittance of approximately 99.8% for a light 193 nm in wavelength, and that it is highly homogeneous so that it provides excellent imaging properties, as described in Japanese Patent Laid-Open No. 53432/1998. The homogeneity of a synthetic quartz glass is generally achieved by applying a homogenization treatment to a synthetic quartz glass ingot obtained by fusion and vitrification of a product obtained by flame hydrolysis of a high purity silicon compound and the like. However, because the synthetic quartz glass ingot is exposed for a long period of time at high temperatures in the homogenization treatment, there occurs contamination due to the impurities generated from the refractories such as alumina, zirconia, graphite, etc., which constitute the furnace material. The loss of transmittance due to the contamination is found particularly noticeable in the case where ArF excimer laser radiation is used; thus, it is unfeasible to use the synthetic quartz glass impaired in transmittance due to this contamination as the optical materials for steppers employing an ArF excimer laser as the light source.
Accordingly, the present inventors proposed a method for recovering the loss in transmittance of a synthetic quartz glass contaminated by the aforementioned homogenization treatment in Japanese Patent Application No. 2762188. In accordance with the method described in that Japanese Patent Application, it has been found that the transmittance for ArF excimer laser radiation is recovered, and that the internal transmittance for a light having a wavelength of 193 nm is recovered to about 99.8%. However, the products thus obtained were not always the same, and it was hence difficult to maintain a stable production of synthetic quartz glass for use in ArF excimer laser lithography. On the other hand, in Japanese Patent Laid-Open No. 53432/1998, there is proposed a synthetic quartz glass for use in ArF excimer laser lithography, which is obtained by a method comprising achieving homogeneous refractive index during synthesis and without applying secondary heating treatment such as homogenization, because several parts per billion of Na are mixed into the glass during the secondary heat treatment. The synthetic quartz glass described in Japanese Patent Laid-Open No. 53432/1998 has an Na concentration of 20 ppb or less, and if the Na concentration is more than 20 ppb, 5 to 100 ppb of Al is required to be contained (see paragraphs [0017] to [0019]. However, because a synthetic quartz glass is produced by depositing silica soot which is generated by flame hydrolysis of silane used as the starting material, followed by fusion and vitrification, superior homogeneity within the plane vertical to the direction of growth of the synthetic quartz glass (i.e., the longitudinal direction) can be readily achieved, however, it is technologically extremely difficult to increase the homogeneity in the direction parallel with the direction of growth (i.e., the lateral direction), because the stripes that generate with the growth, i.e., the so-called layers or layer-structure, are formed in this direction. Furthermore, the production method described in the unexamined published Japanese Patent Application mentioned above makes the apparatus very complicated and expensive, because it requires, in addition to the rotation of the target plate, operations such as vibrating and pulling down as well as the operation of maintaining the synthetic plane and the burner apart from each other at a constant distance. Moreover, the diffusion constant of Al in quartz glass is as small as 1.times.10.sup.-13 cm.sup.2 /sec, which is extremely lower than the diffusion constant of Na, i.e., 7.9.times.10.sup.-6 cm.sup.2 /sec (refer to "Handbook of Glass Properties" (Academic Press)). Thus, it is technologically difficult to homogeneously dope both Al and Na which greatly differ from each other in diffusion constants.
In light of the aforementioned circumstances, extensive studies were conducted on the development of synthetic quartz glass for use in ArF excimer laser lithography having high homogeneity and high transmittance. As a result, it has been found that the production method proposed in Japanese Patent No. 2762188 is most suitable from the viewpoint of its ease in carrying out and its low cost. Further studies have been performed on that production method. It has now been found that the fluctuation of the product quality is attributable to the concentration of Na contained in the synthetic quartz glass, and that by specifying the range of Na concentration and by irradiating at an ultraviolet radiation 260 nm or shorter in wavelength at a specified luminance and for a specified time duration, it is possible to maintain a stable production of a synthetic quartz glass having high homogeneity and yet high transmittance for an ArF excimer laser radiation.